The neonatal period is a time of extensive growth and maturation of the intestinal villus epithelium. The maturation involves changes in membrane composition, mucosal permeability, and digestive enzyme synthesis and secretion rates. This process is dependent on enteral nutrition and is greatly affected by the lipid composition of the neonatal diet. High fat content in the lumen compromises villus epithelium integrity, and the neonatal epithelium is more sensitive than the adults to this insult. An extreme example of this damage is necrotizing enterocolitis which affects primarily preterm and low birth weight infants. Although the etiology of this disease is not clear, it has been noted that formula-fed infants have a higher risk that breast-fed infants of developing this disease. The goal of this proposal is to test the hypothesis that bile salt-stimulated lipase (BSSL), a major protein constituent of breast milk, protects the neonate from lipid-induced injury of the gut epithelium. The milk BSSL is identical to pancreatic cholesterol esterase (CEH) and may serve a similar function before onset of CEH biosynthesis in mature pancreas. Preliminary results show that a BSSL/CEH inhibitor induced significant damage to the villus epithelium of neonates as compared to controls. This application proposes to use a mouse strain with targeted disruption of the BSSL/CEH gene as an animal model to study lipid-induced injury of the neonatal villus epithelium and the restitution of gut integrity. Female CEH knockout mice will be used to nurse CEH (-/-)pups to test the hypothesis that BSSL/CEH is necessary for proper neonatal gut maturation. The nature and time course of the injury and restitution will be established by analyzing histologically and morphometrically the epithelium of control, damaged, and regenerating intestine. Results of preliminary studies predict that these pups will sustain severe mucosal injury unless BSSL/CEH is provided in their milk. An additional hypothesis to be tested is that damage to the villus epithelium of neonates, due to BSSL/CEH deficiency, is caused by accumulation of intermediate metabolities of incomplete lipid digestion. These undigested lipid intermediate metabolites of incomplete lipid digestion. These undigested lipid intermediate metabolites serve as second messengers to alter normal development of the intestinal epithelium, before establishment of the permeability barrier. These studies are important in defining optimal nutritional requirements for growth and development of the neonatal intestine, especially in premature and low birth weight infants.